UPS= uninteruptible power supply.

A self contained unit consisting of batteries, battery charger, inverter, and automatic changeover circuitry.

Commonly used between the wall socket and some important electrical load such as a PC.
In case of power failure it will continue to power the load for perhaps 10 minutes.

By use of smaller loads a longer run time may be obtained.

Very large UPS units need installing by a licensed electrician.
Small ones simply need plugging in.